1. Field of the Invention
The present invention relates to a light wave interferometer apparatus, especially which is used for measuring an aspherical surface shape of an aspherical lens.
2. Description of the Related Art
Recently, there is a strong demand for measuring an aspherical surface shape of an aspherical optical element with high accuracy, especially in the field of designing and manufacturing lenses.
Concerning the technique related to a measurement method of measuring an aspherical shape with high accuracy, in the Fizeau type interferometer, a reflection element for reference having an aspherical surface for reference, which is a basis used for measuring an aspherical surface to be measured, is arranged close to the aspherical surface to be measured. According to interference fringes obtained by light interference of a reference light, which reflects on the reflection element for reference and returns to the aspherical surface to be measured, with object light, which reflects on the aspherical surface to be measured and a shape of the aspherical surface to be measured is measured. At the time of this measurement, interference fringes are scanned. A so-called interference fringes scanning method is known (JP-A-2004-532990).
Further, concerning the technique related to a measurement method of measuring an aspherical surface shape with high accuracy, a so-called spot scanning method, which is disclosed in JP-A-8-146018 and JP-A-2001-133244, is known. Furthermore, a measurement method described in U.S. Pat. No. 6,956,657, in which a synthetic aperture method is used, is known.
However, in the method described in JP-A-2004-532990 and others, the measurement is greatly affected by a dislocation between the optical axis of the aspherical surface for reference (the basis aspherical surface) and that of the aspherical surface to be measured. Therefore, it is impossible to obtain excellent interference fringes with respect to all regions of the aspherical surface shape to be inspected at the same time. After all, in order to obtain the information of interference fringes with respect to the whole surface to be inspected, it is necessary to repeat image-pickup each time the information of interference fringes appears in each region. Further, it is necessary to combine a large number of pieces of information of interference fringes with each other. Therefore, operation of obtaining the information of interference fringes becomes very complicated.
Accordingly, it takes a very long measurement time to execute the methods described above.
Further, in the case of JP-A-2004-532990, a manufacturing cost of the apparatus is expensive. In the case of U.S. Pat. No. 6,956,657, a structure of the apparatus becomes complicated.